The differences between programming on demand service and traditional programming service are well illustrated in the comparison of video on demand service to traditional broadcast television service.
Video on demand service allows each television viewer, or "subscriber" to see a program they want whenever they want to see it. This stands in contrast to traditional broadcast television service which requires each viewer to select from a list of programs that are to be shown at a predetermined time. While video on demand service offers viewers greater selectivity than traditional television service, it poses a significant problem to service providers. The required number of video playback paths, or "video feeds" required in a video on demand system is significantly greater than the number of feeds required in a traditional system.
In current video on demand systems, a separate video feed is required to service each viewer. This is clearly illustrated in the case of two viewers who want to watch the same program but would like to begin watching it at different times. Consider a first viewer who begins watching a program 10 minutes before a second viewer. The second viewer can not begin viewing the program on the same feed as the first viewer because this would cause the second viewer to miss the first 10 minutes of the program. Thus, to provide "true video on demand" the number of feeds must be equal to the number of viewers.
FIG. 1 shows a comparison of a prior video on demand system to a traditional cable television system. As can be seen from the figure, the traditional system requires only one feed, feed 100, to service five households 102a-e, while the prior video on demand system requires five feeds 110a-e to service five households 112a-e. Each "feed" may be thought of as video playback device and a coupling associated with that device for delivering the device's output to a viewer's television set. Examples of video players include: Video Cassette Recorders, laser disk players, and digital playback devices. Examples of couplings suitable for use with such playback devices are: coaxial cables, fiber optic cables, twisted shielded pair cables, and wireless links.
Referring further to FIG. 1, it can be seen that a transmission center for the traditional system 104 includes a video player 116 and a transmission device 108. By contrast, a transmission center for the prior video on demand system 114 includes five video players 116a-e and a transmission and switching device 118, the switching device being necessary to couple each of the feeds to the appropriate player.
The one feed per viewer requirement of prior video on demand systems places a greater hardware burden on both the providers of programs and the operators of the networks over which those programs are transmitted. The amount of hardware necessary to implement prior video on demand systems effectively multiplies the amount of hardware needed to provide programming service by the number of viewers. The attendant costs of maintaining that additional hardware is correspondingly increased. Furthermore, the simultaneous transmission of numerous versions of a program over a network requires more channel capacity than does the transmission of a single version of the same program, and therefore places a greater burden on network resources. The increased burden that video on demand service places on program providers and networks, and the accompanying increased costs, can be reduced if the number of required feeds can be reduced.
Attempts have been made to reduce the number of feeds needed to provide video on demand service. One method is to implement a compromise between true video on demand and traditional broadcast television. If viewers are willing to wait up to .tau. seconds for a program to begin, a program of length T can be distributed using T/.tau. feeds, regardless of the number of viewers who request the program. This method is analogous to broadcasting the program in the traditional manner every .tau. seconds. However, even if viewers are willing to wait some time for their program to begin, they are likely to prefer smaller waiting times when given a choice. Therefore, in an environment where several service providers are offering video on demand, viewers are most likely to subscribe to the provider that offers the smallest waiting time. Thus, competition for subscribers dictates that service providers minimize waiting times.